Stock Car Dynamics, Part III

Understanding race car chassis dynamics helps us set up our stock cars. As we understand how all of the forces work in our suspension systems, we can better adjust the components on the car to create a more balanced setup. In the first part of this series, we learned about the early research of chassis dynamics and the individuals and companies involved. The thread of technology pursued in our past only went so far.

From day one, racers have had to use trial-and-error testing to sort out their setups. Even in Detroit, new cars were taken to a skid pad, which is a circular test pad, in order to determine handling characteristics. Numerous books have suggested racers should use that very same method to tune the setups in their cars.

Previous technology, such as the roll axis and roll couple distribution methods, did not present the entire picture of what was happening with the race car chassis. The early research did define certain characteristics of suspension systems and tires that would lay the groundwork for further research.

2/11A balanced setup is one where two suspension systems in the car want to do the same thing as we drive through the turns. Doing the same thing means they are trying to roll to the same angle. This balance causes the setup to be fast and consistent, both of which are qualities of a winning setup.

In the '90s, a new thread of technology was pursued. A method that addressed the goals racers have for analyzing the chassis setup came from that research and extensive development. It gave us a way to predict the desires of a race car's suspensions. We found that once we knew the measure of each suspension system, we could then change components to cause the car to be balanced. A balanced setup, we learned, helps a car be fast and consistent and use most of the capability of its tires.

A balanced chassis--where both ends are working together--is what we always tried to find. In the past, when we accidentally found a really good setup, we didn't really know how we got there or why it worked so well. Often, the performance could not be duplicated when we built, bought, or otherwise acquired a new car. Part Two Part Two of this series explained all of the elements that influence the chassis setup as far as balancing the two suspension systems. We discussed the moment center (MC) (previously called the roll center), how the moment arm works, and the differences between the front and rear suspension systems. We explained how we can make the car balanced by predicting what the front and rear suspension systems desire to do and then changing components to cause them to work together. That is the most important consideration related to handling performance that we can concentrate on for our race car. To do that, we need to have some kind of idea of what changes to make in order to accomplish that goal.

Sample Setups

3/11The moment arm length determines much of the front suspension dynamics, and its overall design can be the difference between a car that turns well and is fast in the turns and one that pushes and must slow down to get through the middle of the turns. Knowledge about the true picture of the moment arm and its relationship to the moment center has increased dramatically over the past few years.

Now, we will take a look at three commonly raced types of stock cars and see how we can produce a more equally balanced setup for each one. The primary influences on the base setup of the car are the front moment center location, the rear moment center height, and the four spring rates. As we discuss the setups in these cars, follow the reasoning and methods. When you think about making changes to your car, you can move in a direction that makes sense and causes a more positive result.

A Caution

Every race car is built a little differently than another in the same class or division. The examples given are for a specific car (these are real setups that have been used in real racing situations) and probably will not work exactly right for your race car. We are trying to show tendencies for how to balance your setup, directions to go in making positive changes, and why we are doing them. For setting up your car, apply the reasoning, not the specific changes or setups.

Setup information will include the four spring rates (LF, being the left-front spring, shown in the left upper corner of the chart; RF in the right upper corner; LR in the left lower corner; and RR in the right lower corner). The front moment center is shown as height off the ground/width from a centerline that is halfway between the two tire contact patches (a minus number represents an MC that is left of centerline). The Panhard bar is listed as left/right mounting heights off the ground in inches. The banking angle is shown in degrees and the last number shown is the estimated g-forces.

Example One--stock clip Late Model on a dirt track

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With the current values, this car would be very tight and would require a very low amount of crossweight percentage in order to make the car neutral. The two ends of the car are very different in what they desire to do. There's approximately 2.5 degrees difference in desired roll angle.

To balance this car, we could do several things: 1. Soften the front springs and reverse split the rates to: LF = 850, RF = 800. 2. Reduce the rear spring split to: LR = 185, RR = 175. 3. Raise the Panhard bar to 11/12. 4. Move the dynamic width of the MC to 2 inches left of centerline.

With these changes, the two suspension systems are now much closer to being balanced. The car will turn much better and we can use more crossweight percentage that will load the LR tire, which will promote better bite off the corners.

Example Two--stock clip Late Model on an asphalt track

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As with the dirt stock clip car, this setup is a tight one. Because of the high g-forces encountered on asphalt, the front and rear roll angles are more than 3 degrees different, resulting in a very unbalanced setup.

To balance this car, we could make these changes: 1. Change the front spring rates to: LF = 850, RF = 850. 2. Change the rear spring rates to: LR = 175, RR = 175. 3. Raise the Panhard bar to 11.5/12.5. 4. Move the dynamic MC to 3.0 inches in height and 4.0 inches right of centerline. This improves the camber change on the RF wheel and makes the front end more efficient to work harder to turn the car.

These changes will make both ends of this car perfectly balanced, and the performance will be consistently fast because both ends are now working together.

Example Three--an IMCA-type Modified on a dirt track

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With the low g-forces, small moment arm caused by the MC being located too far to the right of centerline, and the stiff front springs, this car is both unbalanced and much too stiff for dirt racing. We can make changes similar to what we did to the stock clip dirt Late Model, which are:

These changes do two important things: By softening the front springs, raising the Panhard/J-bar, and moving the MC to the left, we were able to bring the roll angles of the front and rear closer together to better balance the two suspension systems. We have also increased the rear spring split, which gives us much more bite off the corners.

Example Four--an IMCA-type Modified on an asphalt track

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The asphalt Modified experiences more g-forces, not only from the improved grip that the asphalt gives, but also because of the 14-degree banking of the racetrack and low center of gravity of these cars. Nonetheless, this setup is also very unbalanced. The MC is too far to the right of centerline and the Panhard bar (rear MC) is too low. There is about a 2-degree difference in how far each suspension wants to roll with the rear out-rolling the front. Here is what we might change to correct the setup:

1. Change the front spring rates to: LF = 750, RF = 800. 2. Raise the Panhard bar (rear MC) to: 10.75/11.75. 3. Move the dynamic MC width to 6.0 inches right of centerline. With these changes, the car is now balanced with both ends (suspension systems) wanting to do the exact same thing in the turns. This car will now turn well, drive through the middle faster, and exit the corner much faster.

Example Five--Four-bar Super Late Model on dirt

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This car is set up based on recommendations published by leading dirt Late Model builders about five years ago. Since that time, most dirt Late Model manufacturers have changed their view of a starting setup. Most now publish starting front spring rates that show the LF stiffer than the RF spring. This reverse split helps the car on turn entry.

The older "base" setup makes the cars way too tight. Many times, the driver would need to throw the car sideways in order to have any chance at all of setting up a good line for coming off the corner.

We always try to move the upper mounts of the springs on a four-bar car out toward the wheels as much as possible to widen the rear spring base and help control the excess rear roll tendencies that these cars have.

We can make the following changes to help the car turn better and make it more neutral in the middle of the turns so we don't have to be nearly as sideways:

1. Soften and reverse split the front spring rates to: LF = 400, RF = 375. 2. Reduce the rear spring split to: LR = 200, RR = 175. 3. Raise the right side J-bar to 11.50. 4. Move the dynamic MC to (-) 4.0 inches left of centerline. 5. For tacky/tight track conditions, we would even up the rear springs and level out the J-bar.

The plan for setting up all dirt cars is to narrow the gap between roll angles, front and rear, but not necessarily match them exactly like we do on asphalt cars. This method provides enough front grip so the car will turn well, but will still allow adequate rear bite so the car has enough rear grip to get off the corners.

On tracks where there is plenty of moisture and the grip level is high, we can set up the dirt Late Model more like an asphalt car with even spring rates in the rear and a higher average J-bar. The World 100 and the Dream dirt Late Model races run at Eldora (Ohio) Speedway were won during the last five years using setups with even springs in the front and rear. One car had a pair of 375-pound springs in the front and the equivalent of 175-pound springs in the rear (this was a swing arm car with 350-pound rear springs mounted on the trailing arm--the motion ratio causing the car to feel about half the installed spring rate).

Example Six--asphalt touring Late Model on a flat track

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This car has a front MC that is too far to the right of centerline for a flat asphalt track, stiff front springs, and the rear spring split is too much. The roll angles are 1.8 degrees different and that makes for a very unbalanced and inconsistent setup. This car might be fast for a few laps, but in the long run, the lap times will fall off quite a bit.

These changes will balance the car while still providing sufficient bite off those flat corners. A high crossweight percent range works best to help with exit performance.

1. Change the front spring rates to: LF = 350, RF = 325. 2. Change the LR spring to: LR = 185 (the split is reduced to 10 pounds versus the original 25 pounds). 3. Raise the Panhard bar to: 9.50/10.75. 4. Move the dynamic MC width to the left to end up at 4.0 inches right of centerline. 5. Use a high crossweight range (more equal to the left side weight percentage).

The reverse spring split on the front helps the car on entry to the corners. If we were to run a stiffer RF spring than the LF, the car would begin to roll to the left on braking into the corners because of the softer LF spring. As the car continues to drive farther into the turns, the front must reverse and roll to the right. This gives the driver a very uncomfortable feeling that can best be described as a "flip-flop" sensation.

When we change to running a stiffer LF spring, as we brake into the corner the car will begin to roll the same direction we normally would see at mid-turn. The transition is smooth and the driver will have more confidence on entry.

The rear spring split with the softer RR spring helps promote traction in the rear by loading the crossweight percent as weight is transferred to the rear upon acceleration. Usually a 10- or 15-pound split is sufficient to provide the desired effect where a split of 25 or more pounds is often too much.

The Panhard/J-bar can remain relatively low because of the wider rear spring base that is a characteristic of these cars. On a solid-axle rear suspension like we use in stock cars, the wider the spring base (farther apart the rear springs), the more resistance to roll the car will have at the rear suspension. To balance this type of car versus a stock type of Late Model, we need to run a lower rear MC (which produces a longer rear moment arm) to compensate for the wider spring base.

The front MC change (moving the MC to the left) makes the front suspension "feel" softer and more efficient and want to roll to a greater roll angle to exactly match the rear suspension desired roll angle.

Example Seven--asphalt touring Late Model on a high-banked track

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Many teams who run the flatter tracks mostly will try to set up their cars the same way when at the higher-banked racetracks. The higher banking produces much more downforce, and the arrangement of springs that works best on the flatter tracks won't work well on the high banking. The higher the banking, the worse it gets. Rear spring split with the LR stiffer than the RR cannot be used because the higher g-forces cause more downforce and magnify the effects of the spring split. This makes the cars very unbalanced.

At the high-banked racetracks, we experience more chassis dive and less chassis roll. Excess camber change is a result, but less detrimental to traction because of the high amount of downforce and increased traction caused by more force being exerted on each of the four tires. Here are the changes we would make to the setup to balance the car for a 14-degree track:

1. Reduce the front spring split to: LF = 375, RF = 400. 2. Even up the rear springs: LR = 200, RR = 200. 3. Raise the RR Panhard bar to 11.50 (which raises the rear MC and increases the split in the two ends of the bar to compensate for the increased travel on the RR corner due to the added downforce. 4. Change static cambers on the front--increase the LF positive camber and decrease the RF negative camber. 5. Use a low crossweight range (from about 50 to 52 percent).

Example Eight--Asphalt Touring Late Model on a very high-banked track

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The following is where we would want to be if the track banking angle was much higher, at say 26 degrees. The car will have a lot of traction due to the high amounts of downforce and therefore the cornering speeds will be high. The resultant force (a combination of gravity and the lateral cornering force called centrifugal force) will be in a direction that lies between the front tires. This results in a lot of vertical chassis travel and little chassis roll. Traction is never a problem unless the setup is very unbalanced. A balanced setup for a very highly banked race track for an asphalt Late Model.

Note that the front spring rates are much higher and the rear spring rates are both higher and split to a greater extent. Both of these could be even higher if the radius of the turns is smaller. The front MC is farther to the right, the average Panhard bar height (rear MC height) is higher, and the g-forces are up considerably to 2.2 g's. The front suspension travel will average in excess of 3 inches and the rear suspension will travel upwards of 5 inches or more. If your car normally has a 4-inch ride height, then the car must be raised or it will make contact with the track surface.

The starting cambers might be: LF = +3.5, RF = (-) 1.0. That is because of the high amount of chassis dive the car will experience that will reduce the positive LF camber (become less positive) by 3 or more degrees and increase the (add to the negative amount) RF camber by about the same amount.

Conclusion

Again, let me say the changes we have made to these sample cars are the same as we have made to real cars in actual competition. It doesn't mean you need to run out to your shop and put these setups in your car. Remember the directions we have taken and trends we have described related to the different types of race cars. The hot tire temperatures, as well as the tire wear, will be an indication of how close you come to a balanced setup.

The differences between these cars and yours that might cause these setups not to work for you include: a) center of gravity height; b) front moment center (roll center) location; c) front end motion ratios for spring mounting; d) weight distribution; and e) front and rear spring location and spring angles.

All race teams need to know certain basic information about their cars. Learn the tendencies that make a car's suspension want to roll more or less and try to match the ends for a more balanced setup. Know your front geometry settings, especially the moment center location and camber change characteristics. If you do, finding that sweet spot related to a winning chassis setup will be a much easier process. Above all, be willing to take control of your chassis, and do not be afraid to make changes to improve the geometry, the alignment, and the balance of your setup.